The present invention relates generally to wireless communication devices, and in particular to a transmitter portion of a wireless communication device.
The frequency spectrum that is shared among radio communication devices is limited. Thus the ability of a transmitter to transmit as much information as possible in an allocated frequency spectrum or channel without interfering with other communication devices in adjacent channels is of great importance. To transmit as much information as possible in the allocated channel, digital communication systems typically modulate both the amplitude and phase of a radio frequency (RF) carrier. The amplitude modulation allows more information to be encoded on the carrier in a given channel than if only the phase was modulated. However, the amplitude modulation puts additional requirements on the transmitter that would not exist if only the phase of the RF carrier was modulated.
These additional requirements are due to the inherent nonlinear effects resulting from the amplification of an amplitude modulated signal by an RF power amplifier. Due to the nonlinear characteristics of the RF power amplifier, signal distortion components that include an amplitude component and a phase component are added to the original signal. These additional components are due to the amplitude compression characteristics (AM/AM) and the phase distortion (AM/PM) characteristics of the RF power amplifier when it is driven over a range of amplitudes. If these distortion components are not compensated they will cause spreading of the spectrum into the adjacent channels and thus interfere with communication devices using adjacent channels.
A number of prior art signal processing techniques have been developed to compensate for the nonlinear characteristics of RF power amplifiers. One such technique involves the use of a feed forward correction circuit in a feed forward amplifier. In general, feed forward amplifiers separate out distortion components generated by the RF power amplifier to create an error signal. The error signal is then amplified and added to the RF power amplifier""s output with an amplitude, phase, and delay adjusted for maximum cancellation of the distortion components. However, the amount of distortion reduction available in a feed forward amplifier is limited by the distortion introduced into the error signal when the error signal is amplified by an error amplifier.
For example, FIG. 1 is a block diagram of an exemplary feed forward amplifier 100 of the prior art. Feed forward amplifier 100 includes a main signal path 102, a feed forward correction circuit 104, and a control circuit 106. An input signal 101 having carrier components is sourced to main signal path 102, where the signal is routed to a gain and phase adjuster 110 via an input signal coupler 108. Gain and phase adjuster 110 adjusts the amplitude and phase of input signal 101 based on a control signal received from control circuit 106. Gain and phase adjuster 110 conveys the amplitude and phase adjusted input signal to a radio frequency (RF) power amplifier 112 that amplifies the signal to produce an amplified signal 113. RF power amplifier 112 then conveys amplified signal 113 to a first output signal coupler 120 via a signal coupler 114 and a delay circuit 116. As mentioned above, RF power amplifier 112 introduces distortion components to the amplified signal, which distortion components are partially cancelled by an error signal output by feed forward correction circuit 104.
Feed forward correction circuit 104 produces the error signal based on input signal 101 and amplified signal 113. A summation junction 124 included in feed forward correction circuit 104 receives a portion of input signal 101 via input signal coupler 108 and delay circuit 122 and further receives a portion of amplified signal 113 via signal coupler 114. Summation junction 124 subtracts the received portion of the amplified signal from the received portion of the input signal to produce an error signal 125. The subtraction results in a partial cancellation of the carrier components of the received portion of amplified signal by the carrier components of the received portion of the input signal. As a result, error signal 125 primarily contains the distortion components of the received portion of the amplified signal.
Summation junction 124 conveys error signal 125 to a feed forward correction circuit error amplifier 130 via a feed forward signal coupler 126 and a feed forward gain and phase adjuster 128. Error amplifier 130 amplifies the received error signal to produce an amplified error signal 131 and conveys the amplified error signal to first output signal coupler 118. First output signal coupler 118 combines amplified error signal 131 with amplified signal 113 to partially cancel the distortion components of amplified signal 113 and produce a distortion reduced output signal 121.
Amplification of error signal 125 by error amplifier 130 may result in an introduction of distortion components to the error signal due to the amplitude compression and the phase distortion characteristics of the error amplifier. Since amplified error signal 131 is combined with amplified signal 113 at output signal coupler 120, it is desirable to minimize the added distortion. In order to reduce the distortion introduced into amplified error signal 131 by error amplifier 130, control circuit 106 controls an average power of an error amplifier drive signal, that is, error signal 125.
Control circuit 106 receives a portion of error signal 125, that is, attenuated error signal 127, from feed forward signal coupler 126. Control circuit 106 further receives a portion of output signal 121, that is, attenuated output signal 132, from a second main signal path output signal coupler 120 that receives output signal 121 from first main signal path output signal coupler 118. Control circuit 106 routes each of attenuated error signal 127 and attenuated output signal 132 to a switch 140, typically a multiplexer.
Switch 140 is controlled by a controller 148 coupled to the switch. When switch 140 receives a first switch control signal 152 from controller 148, the switch routes attenuated error signal 127 to a mixer 142. A local oscillator 150 coupled to mixer 142 and controller 148 sources a reference signal to the mixer. In response to a first local oscillator control signal 149 sourced by controller 148 to local oscillator 150, the local oscillator adjusts a frequency of the reference signal such that mixer 142 downconverts attenuated error signal 127 to baseband to produce a baseband error signal. Mixer 142 then conveys the baseband error signal to an average power detector 146 via a band pass filter 144 coupled to the mixer. Average power detector 146 determines an average power of the baseband error signal. Controller 148 reads the average power determined by average power detector 146 and, based on the average power, conveys a control signal 156 to gain and phase adjuster 110 that is designed to minimize the average power of the baseband error signal. In response to receiving control signal 156, gain and phase adjuster 110 adjusts an amplitude and phase of input signal 101, thereby adjusting an average power of error signal 125 and adjusting the average power of the baseband error signal.
When switch 140 receives a second switch control signal 154 from controller 148, the switch routes attenuated output signal 132 to mixer 142. When attenuated output signal 132 is routed to mixer 142, controller 148 sources a second local oscillator control signal 151 to local oscillator 150. In response to the second local oscillator control signal 151, local oscillator 150 adjusts the frequency of the reference signal such that mixer 142 downconverts attenuated output signal 132 to baseband to produce a baseband output signal. Mixer 142 then conveys the baseband output signal to average power detector 146 via band pass filter 144. Average power detector 146 determines an average power of the baseband output signal. Controller 148 reads the average power determined by detector 146 and, based on the average power, conveys a control signal 158 to feed forward correction circuit gain and phase adjuster 128 that is designed to minimize the average power of the baseband output signal. In response to receiving control signal 158, gain and phase adjuster 128 adjusts an amplitude and phase of error signal 125, thereby i) adjusting an average power of error signal 125 and the cancellation the distortion components of amplified signal 113 by amplified error signal 131, ii) reducing the distortion components of output signal 121, and iii) adjusting the average power of the baseband error signal.
Control of an average power of error signal 125 does not necessarily minimize the distortion introduced into the amplified error signal by error amplifier 130, and therefore does not necessarily minimize the distortion included in output signal 121. By controlling the average power of the error amplifier drive signal, the prior art provides sub-optimal reduction of distortion introduced by feed forward correction circuit 104 as the prior art fails to minimize the peak power of the error amplifier drive signal, which is based on error signal 125, and the distortion introduced into an amplified error signal by the peak power of the error signal.
Therefore a need exists for a method and apparatus for minimizing the distortion introduced into a feed forward amplifier by the feed forward correction circuit.